It is common in the automobile industry to use springs to alleviate the muscular effort required to raise heavy car body panels, such as engine hoods, hatchbacks and trunk lids. Moreover, it is customary to replace such mechanical springs which have become worn with cylinder-and-piston telescoping apparatus called pneumatic springs or gas springs. A description of the construcion and operation of such springs is given in U.S. Pat. No. 4,089,512 to Allinquant et al.
With respect to these gas springs, the force thereof is a function of the gas pressure therein, which is called in the art "inflating pressure". At the same time, the gas filling the cylinder is subjected to Boyle's law, which amounts to saying that the bearing effort will, mutatis mutandis, be much higher on sultry days than during very cold weather. Since it is necessary that, under all circumstances, a person even of limited strength should be able to close the hood or the hatchback of the car against the action of the gas spring, the manufacturer will have to determine the inflating pressure by taking into account the highest temperatures likely to be encountered, say during the hot hours of a summer day when the car has remained exposed to the sun.
On the other hand, during reverse meteorological conditions, i.e., during cold weather, the gas spring will prove "slack" and possibly even too weak to maintain the hood or hatchback in its raised position, which can then unexpectedly fall and cause injury to the person leaning at that moment over the engine or the trunk.
It has already been proposed to overcome this danger by providing the gas spring with a mechanical system for locking it in its extended position which corresponds to the open position of the hood or hatchback.
Such prior art systems are described in U.S. Pat. No. 3,947,006 to Bauer et al., as well as French Pat. No. 2,347,575 to Automobiles Peugeot, which patents both call for an annular groove forming a recess in the cylinder wall at the adequate axial level which will be engaged by a resilient lock member associated with the piston when this member reaches this axial level. Such prior art arrangements, while effectively meeting the specified object, have nevertheless a few drawbacks:
during mounting of the apparatus in the workshop, it is necessary to radially inwardly compress the resilient member in order to introduce it into the cylinder and to secure it to the piston. This requires sophisticated machinery and renders difficult the automation of the mounting process;
in use, the resilient member exerts permanent friction on the cylinder wall all along its length, which friction unduly increases resistance and prematurely wears out the essential parts.
An object of the present invention is to provide improvements to such prior art systems by facilitating automated production of the gas spring, improving its operative conditions and increasing its life.